China's Internet population reached 564 million at the end of December, an increase of 26 million over the past six months, according to a non-profit research group in the country.

The new figure brings China's Internet penetration to 42.1%, said the government-linked China Internet Network Information Center (CNNIC) in a report on Tuesday. In contrast, the U.S. has an Internet penetration rate of 77%, according to Internet World Stats.

China's Internet population ranks as the world's largest, but user growth has slowed in recent years, a trend that is not expected to change, according to the CNNIC. Among non-Internet users surveyed in December, 77.3%t said there was either no likelihood or little likelihood they would go online in the next six months. This marks an increase of 12 percentage points from mid-2011.

CNNIC said the main reasons are because many non-Internet users still struggle to understand computers and the Internet. Many of these users have little need to go online, and current Internet applications have yet to intersect with their lives.

To open the Web to more Chinese, the CNNIC advises the industry go beyond building more networks and lowering product costs, by coming up with innovative ways to conveniently bring Internet services to different user segments.

More users in the country are going online with mobile phones. At the end of December, China had 420 million mobile Internet users, adding 32 million users over the six month period. Helping to drive the growth are low-cost smartphones, cheaper data plans, and the popularity of mobile apps. The handsets are also helping users in rural China and low wage earners to go online.

81 percent of China's Internet users are aged 39 and below, with those aged in their twenties making up the largest group at 30.4%.

As for Internet users' level of education, 35.6% of China's Internet users have only attained middle-school education or less.

CNNIC counts Internet users as those aged six and up, who have gone online in the past six months. This has drawn criticism that the group's statistics are inflated.